1. Technical Field
The disclosure relates to a metal separator for a fuel cell.
2. Description of Related Art
A cell of a solid polymer electrolyte fuel cell is provided with a membrane electrode assembly (MEA) constituted by an ion permeable electrolyte membrane and electrode catalyst layers (electrode catalyst) on an anode side and a cathode side with the electrolyte membrane interposed therebetween, a gas diffusion layer (GDL) for accelerating gas flow and improving current collection efficiency is provided on the outside of each electrode catalyst layer, and a metal separator is provided on the outside of the gas diffusion layer. The metal separator defines each cell. Gas or a cooling medium flows in groove channels of the metal separator. A fuel cell is formed by stacking a basic number of cells corresponding to required power.
In the fuel cell described above, hydrogen gas or the like as a fuel gas is supplied to the anode electrode, and oxygen or the air as an oxidant gas is supplied to the cathode electrode. In each electrode, gas flows in an in-plane direction via a unique gas channel layer (expanded metal, a sintered metal foam body, or the like) or the metal separator, and gas diffused in the gas diffusion layer is then guided to the electrode catalyst layer such that an electrochemical reaction is induced.
The metal separator is described in more detail. A groove channel through which the gas flows is formed in a linear shape or meandering shape on one side, and a groove channel through which the cooling medium flows is formed on the other side. For example, the oxidant gas or fuel gas flows in the in-plane direction through the groove channel in the metal separator side surface facing the membrane electrode assembly side (gas diffusion layer side), and in a procedure in which the gas flows in the in-plane direction, the oxidant gas or fuel gas is supplied to the gas diffusion layer such that the oxidant gas or fuel gas is diffused and supplied to the membrane electrode assembly via the gas diffusion layer.
In addition, the example configurations using a so-called flat type metal separator in which the gas channel layer is separated include three-layer structure in which an intermediate layer (intermediate plate or the like) having a channel formed therein is interposed between two plates, and a configuration in which an intermediate layer is formed as a metal or resin frame member and a large number of dimples or ribs that define the channel protrudes from one of the two plates to form a cooling medium channel (this structure may also be included in a metal separator having a three-layer structure). Such metal separator is a metal separator for any one of the anode side and the cathode side of a corresponding sell itself and simultaneously acts as a metal separator for the other of the anode side and the cathode side of an adjacent cell in a state where cells are stacked.
The metal separator described above comes into contact with acidic product water and is at a high potential. Accordingly, the metal separator has problems of gas leakage and cooling water leakage due to pitting corrosion or has inherent problems of deterioration in the electrolyte membrane due to metal ion elution. Therefore, in many cases, as the material of the metal separator, stainless steel with high corrosion resistance is applied.
However, in a case of further increasing the potential to improve the output of the cell and fuel efficiency (for example, an increase in potential from 0.9 V to 1.0 V or higher), there is a possibility that corrosion resistance cannot be guaranteed only by a chromium oxide film of general stainless steel due to dissolution of the chromium oxide film.
This will be described with reference to FIG. 13. FIG. 13 shows the results of experiments conducted by the inventors regarding the total amount of electricity (metal elution amount) in a case of a potential of 0.9 V and the total amount of electricity at a potential of 1.0 V for stainless steels JIS SUS 304, JIS SUS 447, and NAS 354 manufactured by Nippon Yakin Kogyo Co., Ltd. (hereinafter, sometimes denoted only by numbers).
As is apparent from FIG. 13, although metal elution is suppressed by high-alloying of the metal separator at a potential of up to 0.9 V, it is difficult to suppress metal elution even by high-alloying of the metal separator when the potential becomes 1.0 V.
This is because, when the potential becomes higher than 0.9 V, chromium or iron oxide films covering the surface of stainless steel cannot be stably formed.
For example, FIGS. 14 and 15 are correlation diagrams (Ellingham diagrams) of potential versus pH for chromium and iron, respectively, and show regions with usability.
As is apparent from FIGS. 14 and 15, in the regions with usability for both chromium and iron, regions deviating from regions in which anti-corrosion can be expected by the oxide films are present in regions with a particularly high potential.
Therefore, a measure to achieve anti-corrosion by performing a surface treatment on the metal separator is considered. However, when a surface treatment is performed, new possibilities such as scratching and generation of defects on the surface of the metal separator are incurred, and there is a possibility of degradation in the metal separator, which is not preferable.
Here, Japanese Patent Application Publication No. 8-180883 (JP 8-180883 A) relates to a separator for a fuel cell and discloses a technique in which stainless steel or a titanium alloy is applied as a metal material for easily forming a passive film on the surface of the separator.